6 But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel, 7 and not all are children of Abraham because they are his offspring, but “Through Isaac shall your offspring be named.” 8 This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring. 9 For this is what the promise said: “About this time next year I will return, and Sarah shall have a son.”
It’s a rather shocking statement to any Jewish person. Paul is saying that it is not enough to be a biological descendant of Abraham. The people of Israel are that. So he isn’t saying they aren’t of Abraham. But he is saying the true descendants of Abraham in the kingdom of God are those who believe by faith.
Abraham technically had two sons – Ishmael by his physical union with Hagar, Sarah’s servant. And then the miraculous child that was conceived from the promise of God through Isaac. Isaac is actually called in the Torah as Abraham’s “only son”